The majority of the gas discharge light sources of known design and especially the gas discharge lamps comprising mercury as a metal additive have been applied since relatively long time and they can be used especially for lighting purposes on open areas, e.g. on roads, squares etc. In the gas discharge light sources realised with a gas discharge process generated in vaporised mercury the most important source of the optical radiation is the gas discharge arc existing even in a mercury vapor environment. The disadvantage of these lamps is that the optical spectrum of the radiation emitted by the gas discharge arc generated in a mercury vapor system comprises basically only two spectral ranges, i.e. the range from the yellow to the green (the wavelength range from 546 to 577-79 nm) and the range from the blue to violet (the wavelength range 440 to 404, 407 nm). This radiation spectrum is shown in FIG. 1 taken from a gas discharge arc generated in a mercury vapor system.
The color rendition of the light sources radiating the only light generated by a discharge arc in a mercury vapor system is not satisfactory. It can be improved by applying an appropriate luminescent material coating a surface directed to the gas discharge space. In this way a special optical transformation is carried out: the luminescent material absorbs a part of the incident optical radiation generated by the discharge arc and especially absorbs from the ultraviolet part thereof and by luminescing in a selected spectral range, especially in the range of the red color it improves the color rendition by adding a part of required color to the emitted optical radiation.
The luminescent material is adopted generally on an inner surface of an envelope including a space determined by electrodes for generating the gas discharge arc, the space being filled with mercury as a metal additive and a rare gas. The high-pressure mercury vapor gas discharge lamps are equipped with an outer envelope bearing the layer of the luminescent material and including a closed discharge vessel comprising the electrodes and the filling (the metal additive and the rare gas). In the low-pressure mercury vapor gas discharge lamps the outer envelope generally lacks and the gas discharge vessel itself is coated on its inner surface with the luminescent material. The voltage supply of the last is ensured by means arranged in the end parts of the discharge vessel; in the high-pressure mercury vapor gas discharge lamps the outer envelope is completed by a normal standardised socket, e.g. of Edison-type according to the well known solutions accepted by the general practice.
The background art refers to several chemical compositions and blends thereof to be taken into account when preparing luminescent materials for improving the optical radiation spectrum of a gas discharge light source and especially those operating with mercury vapor. The U.S. Pat. No. 2 748 303 discloses a high-pressure mercury vapor gas discharge lamp with luminescent material consisting of magnesium fluorogermanate activated by manganese. The translucent envelope of the lamp is covered by a layer of this luminescent material. It has the disadvantage of the germanates: they are not always as stabilized as required. The U.S. Pat. No. 3,569,762 issued to Levin et al offers a solution of this problem: the luminescent material proposed consists of yttrium vanadate and/or yttrium vanadate phosphate, both activated by trivalent europium. The optical spectrum in the visible range of the light emitted by a high-pressure mercury vapor gas discharge lamp realised with the luminescent material consisted of yttrium vandate activated by trivalent europium-Eu(III)-is illustrated in FIG. 2.
Lots of attempts have been noticed for ensuring a further improvement of the color parameters of the light emitted by a mercury vapor gas discharge lamp:
(1) In the fluorescent lamps of well-known different designs (i.e. in the low-pressure mercury vapor gas discharge tubes) a blend of luminescent materials emitting in three color ranges is generally applied. The color regions of the radiation are: blue, green and red. This solution can be taken over also to the high-pressure mercury vapor lamps as it is comprised in the U.S. Pat. No. 4,431,942. According to the last the blend applied in the correction layer, i.e. in the layer of the luminescent material covering an inner surface of a high-pressure mercury vapor gas discharge lamp should include: strontium chloroapatite or barium magnesium aluminate, each activated by bivalent europium, for radiating in the spectral range from 440 nm to 470 nm (from violet to blue), zinc sulphide activated by bivalent copper or calcium sulphide activated by trivalent cerium or calcium magnesium aluminate activated by trivalent cerium and trivalent terbium, for radiating in the spectral range from 520 nm to 560 nm (from green to yellow) and yttrium vanadate or yttrium vanadate phosphate, both activated by trivalent europium, for radiating in the spectral range from 605 nm to 630 nm. According to the description the light source of this kind can be regulated to have color temperature in the value range from 2700K to 2900K. The practice hasn't accepted this blend as luminescent material.
(2) A two component blend is applied for covering the outer envelope of the high-pressure mercury vapor gas discharge lamps. The first component is the yttrium vanadate or yttrium vanadate phosphate proposed by Levin et al. in the U.S. Pat. No. 3,569,762 and the second consists of an yttrium aluminate of garnet crystallization system (the garnets are mixed oxides forming holohedral crystals of cubic lattice). Yttrium aluminate absorbs the optical radiation of 435 nm wavelength produced by the gas discharge arc in the environment of the mercury vapor transforming it into the spectral range below and in the proximity of 560 nm, ensuring radiation in the yellow-green color region. This solution is shown in the U.S. Pat. No. 4,034,257 to M. Hoffman and U.S. Pat. No. 4,241,276 to Wyner et al. M. Hoffman discloses the luminescent material emitting the red light, i.e. the above mentioned yttrium-vanadate and/or yttrium vanadate phosphate activated by trivalent europium and the yttrium aluminate of garnet crystal structure has to constitute about 5 to 30% of the mass of the resulting luminescent material. The composition of this additive material of garnet crystal structure can be determined by the general structural formula EQU Y.sub.3-x Ce.sub.x Al.sub.5 O.sub.12
wherein the value of the index x lies in the expedient range 0.004&lt;x&lt;0.02.
Wyner et al in the U.S. Pat. No. 4,241,276 disclose that the second component should constitute about 35 to 40% of the mass of the resulting luminescent material. This amount has to be preferred in order to improve the lm/W efficiency of the gas discharge lamps. The visible part of the spectrum of a high-pressure mercury vapor gas discharge light source realised with a luminescent material layer of Wyner et al is illustrated in FIG. 3.
The luminescent material with yttrium aluminate garnet activated by trivalent cerium added in the amounts defined by the background art has double object. The first of them is to absorb the 435 nm (blue) part of the radiation emitted by the discharge arc and thereby to shift the color point of the light source in the direction of the lower color temperatures in the line of the black body radiation resulting in improved color rendition of the lamp, too, and the second of them is to be excited under influence of the incident optical radiation and to emit radiation with wavelength about 560 nm. This is an advantegeous transformation of the incident optical energy. The blue color radiation in the spectral range of 435 nm is absorbed by the cerium(III) atoms present in the crystal lattice of the garnets, forming the trivalent components thereof. The absorption process shows intensity proportional to the amount of the cerium(III) atoms at the beginning. The lower limit can be determined only on the basis that a definite cerium(III) amount is necessary to ensure efficiency as low as detectable. The amount of the cerium(III) ions can not be, however, endlessly increased, i.e. there exists an upper limit whereover the cerium(III) ions can not be the part of the crystal lattice, it is no space for them therein: the excess of the cerium(III) ions constitutes a separate phase. The lower limit of the amount where this separate phase can be detected, can be identified with the upper limit of the acceptable amounts of cerium(III) in the lattice. In the Journal of El. Chem. Soc. (1973, 69, p. 278) it can be found that the excess cerium(III) ions form in the mentioned conditions an aluminate phase (CeAlO.sub.3 ). This phase deteriorates the optical features of the luminescent material, also when it is present in very low amounts. The same refers to the lanthanum(III) ions which can be accepted by the garnet structure lattice of the yttrium aluminate with similar difficulties--they can replace about 1% of the cerium(III) atoms according to Hoffman and Wyner.
The Hungarian patent specification No. 194 649 (corresponding to the NL application No. 85'02025 of 15th July 1985 filed in the name of N. V. Philips Gloeilampenfabrieken, Eindhoven, Holland) discloses that in the low-pressure mercury vapor gas discharge lamps the concentration of the Ce(III) ions may not be higher than 0.15 mol for 1 mol of the garnet because the higher concentration results in forming separate undesired phases.